The present invention relates to surface coating measuring processes and apparatus. In one aspect, this invention relates to an aluminum alloy sheet surface coating measuring apparatus and method.
Quality coated aluminum alloy metal sheet manufacturing processes require measuring and knowing the amount and depths of surface coatings on the aluminum alloy metal sheet.
Conventionally, rapid and capable measurements of organic coating weight are important for manufacturing coated sheet products, i.e., such as coated aluminum metal or metal alloy sheet products. These measurements are important for aluminum metal or metal alloy sheets which are re-oiled, roll coated, conversion coated, or which have coatings applied by the electro-coating process. Quality measurements are needed for providing coating uniformity measurements in units of mils as well as mg/ft.sup.2, mg/in.sup.2, or g/m.sup.2.
Coating aluminum alloy metal sheet involves a number of different coating materials, each coating designed to meet the needs of a particular aluminum alloy sheet product line. Coatings are applied to aluminum sheet using one of two application techniques, electro-coating (E-Coat.RTM.) or roll coating. The application method is dictated by the coating material type or the end product use. In the E-Coat.RTM. technique, the same coating is applied to the top and bottom sheet surfaces. Roll coating allows for the same or different coatings to be applied to the two surfaces.
Quality control of the coating material weight today involves several different analytical techniques. The different techniques include near infrared (NIR) spectroscopy, gravimetric strip weight measurements, and strand gauge measurements. All of these process control methods are quantitative analytical techniques aimed at determining the amount of coating applied on aluminum sheet. Weight measurements are made on designated coils of metal on both the top and bottom surfaces and across the entire sheet width at the operator, center, and drive positions.
Strip weight methodology is the primary measuring standard for determining coating weights and has been used commercially for several years. Strand gauge and NIR measuring techniques are secondary methodologies, calibrated with a set of primary strip weight standards for each coating type. Strand gauge methodology also has been used commercially for several years, while NIR is a relatively new technique.
The strip weight methodology is gravimetric based. The method requires determining sample gross weight, stripping off the coating from one of the metal surfaces, and reweighing the stripped sample. The net difference between the two weight measurements (standardized to mg/in.sup.2 (mg/cm.sup.2) of surface) represents the coating weight for that particular surface (top or bottom) and coil position (operator, center, drive). Because the sample is greater than one square inch (6.5 cm.sup.2), the reported coating weight (in mg/in.sup.2 (mg/cm.sup.2)) represents an average weight over the 7.07 square inches (45.6) (3-inch (7.6 cm) diameter circular disk) sample size.
Depending on the coating type, the strip weight method utilizes one of two techniques that aid in coating removal from the metal substrate, i.e., concentrated sulfuric acid or methyl ethyl ketone (MEK) solvent. Sulfuric acid typically is used to remove water-based coatings. The acid acts by swelling the coating, allowing the coating to be peeled off the metal. MEK is typically used to remove organic-based coatings. The solvent softens the coating by dissolving some of its polymer base. When softened, the coating can be rubbed off the metal with a brush. With both of the techniques, the strip weight method leaves a clean, coating-free metal surface.
The strip weight methodology is time consuming (average of 10 minutes per sample) and involves hazardous operation. The method is destructive in nature and requires punching a three-inch (7.6 cm) diameter circular sample from an appropriately designated coil of coated metal.
Strip weight measurements are hazardous operations from both safety and environmental points of view. Sulfuric acid is a highly corrosive, oxidizing agent that can cause severe burns to the skin or lung tissue, either through direct contact with the acid or with the acid vapors. MEK is a class 1B flammable solvent that is harmful if inhaled, swallowed, or absorbed through the skin. Strip weights should be performed in a properly functioning exhaust hood with the appropriate safety equipment. Both of these stripping agents are also considered hazardous wastes and must be disposed of following established hazardous waste disposal procedures.
The stand gauge methodology for coating weight determinations is a non-destructive secondary quantitative analysis technique. The method provides coating weight measurements as a function of electric current resistance through a coated sample between two probes. One probe is placed on the coating, covering an approximately 0.3 square inch (1.9 cm.sup.2) area. The other probe is placed in direct contact with the underlying metal. Coating weight data obtained from this method represents the average weight distributed over the 0.3 square inches (1.9 cm.sup.2) contacted by the probe. For each coating type, several calibration curve is constructed that relates measured resistance to weight for each of the standards. This calibration curve then can be used to predict coating weights on samples. A separate calibration curve and standard set are needed for each coating type analyzed by this method.
When strip weight standards have been accumulated, safety and environmental concerns using strand gauge methodology are minimal. This technique is capable of predicting strip weights within minutes and within a .+-.0.3 mg/in.sup.2 tolerance with the strip weight value without actually stripping the sample. However, with heavier coating weights (those above 7 mg/in.sup.2 (1.9 mg/cm.sup.2)), strand gauge precision and accuracy errors exceed strip weight data by more than the established 0.3 mg/in.sup.2 (0.05 mg/cm.sup.2) tolerance. Hence, strand gauge methodology is not used to determine the heavier weight coatings.
The NIR (near infrared) spectroscopy measurement methodology is capable of predicting heavy coating weights. Like the strand gauge, NIR methodology is a non-destructive secondary quantitative analysis technique. Analysis time is on the order of only a few minutes per sample, and its mode of operation is based on the response from combination bands of aliphatic C-H stretch absorptions in the near infrared frequency range (2458 manometers). Prediction capability (NIR data vs. strip weight data) is less than 0.2 mg/in.sup.2 (0.03 mg/cm.sup.2).
Coating weight data obtained from the NIR spectroscopy measurement method represents the average weight distributed over approximately a 1 square inch (6.5 cm.sup.2) area. Coating weights are predicted from calibration curves that relate instrument responses to strip weights for each coating type. As with the strand gauge, when step weight standards have been accumulated, safety and environmental concerns are minimal with the NIR methodology.
Since NIR methodology uses normal incidence, this method is less accurate for coating weights on aluminum sheet.